Many of today's modern metallic structures, such as steel bridges and aluminum airplanes, are exposed to cyclical compressive and tensile forces over their useful life. These forces have a plastic component, in which the metal undergoes deformation above its yield point, and an elastic component, in which the metal is stressed at a level below its yield point. The degree to which the metal performs over the years that it is in service is largely affected by the nature of these forces, and the corrosive environment that surrounds the metal. These environments can contain atmospheric conditions, such as acid rain and salt water, as well as man-made corrodants, such as gasoline and acids. The combination of a corrosive environment and cyclic forces creates a damage mechanism commonly referred to as "corrosion fatigue".
There are a number of techniques for measuring fatigue damage of a metal structure. Non-destructive testing, such as dye penetrant inspection, ultrasonic testing, and magnetic particle detection are just some of the more traditional techniques that have been used to determine the presence of cracks in components which have undergone fatigue damage. Although these methods are useful in forewarning catastrophic failure, they rely upon the existence of crack-like defects which are large enough to detect, and cannot perceive any other type of damage caused by cyclic stresses.
Fatigue strain gauges and fuses have also been used to predict fatigue life. Fatigue gauges rely upon monotonic changes in resistance for determining the degree of fatigue. Fatigue fuses are essentially miniature fatigue specimens attached to a structure, which undergo the same cyclical stresses as the structure and provide advanced warning of the development of fatigue damage. Although these devices have practical utility, they require advanced knowledge of an existing fatigue problem and merely provide a cumulative assessment of the damage from the onset of service life. They have little or no value in detecting the current state of damage if they were not previously affixed to the structure prior to service.
Accordingly, there is a need for methods and devices for measuring fatigue damage which can be used at any point during the service life of the metallic structure, from the day it is placed in service, through the point at which it is no longer useful for its intended purpose.
There is a further need for methods and devices which provide a capability for wide area coverage of a surface of a metallic specimen for evaluating the fatigue status of as much of the specimen as desired, while coupled with the ability to identify and monitor the most acutely affected or significantly fatigued areas of the specimen.